This invention pertains to fluid bed catalytic oxyhydrochlorination of ethylene to produce 1,2-dichloroethane, commonly called ethylene dichloride (EDC). It relates specifically to a novel method for improving the fluidized copper catalyst used in such oxyhydrochlorination.
EDC is most easily produced commercially by the direct chlorination of ethylene, and is used in greatest quantity for pyrolysis, or "cracking", to produce vinyl chloride monomer (VCM), on which the vinyl plastic industry depends. The pyrolysis reaction produces, in addition to VCM, by-product hydrogen chloride (HCl) which is advantageously utilized at the plant site to produce more EDC for the pyrolysis. This is accomplished by the process called ethylene oxyhydrochlorination (or sometimes, more simply "oxychlorination") which involves the reaction of HCl with oxygen, supplied as such or as air, and ethylene in accordance with the empirical equation: EQU 2C.sub.2 H.sub.4 +O.sub.2 +4HCl.fwdarw.2CH.sub.2 ClCH.sub.2 Cl+2H.sub.2 O
The ethylene oxychlorination is carried out in many highly successful commercial installations through the world by passing the gaseous reactants at elevated temperature and pressure through a fluidized solid catalyst bed in the manner and under the conditions generally described in Harpring et al U.S. Pat. No. 3,488,398, the disclosure of which is incorporated by reference thereto, as if fully set forth herein. As taught by the Harpring et al patent, in order to achieve sufficient utilization of HCl for formation of EDC, the molar ratio of ethylene to oxygen to HCl is to be maintained in the range of about 1.0 to about 1.2 moles ethylene to about 0.55 to 0.9 moles oxygen for each two moles of HCl, with the most preferred ratio of ethylene to oxygen to HCl being about 1.0 to 0.8/2.0. The process is operated at a temperature in the range of about 190.degree. to 250.degree. C., preferably under a pressure of 10-50 psig, which provides a contact time of from about 10 to about 40 seconds. The reactants are fed into the reactor in a dry state and the pressure-temperature relationships are such that the dew point temperature is always exceeded and there is complete lack of liquid in the reactor. Not more than 0.5% of the catalyst bed is removed from the reactor as fines in a 24 hour period.
The nature of the fluidized catalyst bed is of importance to the success of the ethylene oxyhydrochlorination process. The catalyst bed consists essentially of a copper compound, preferably cupric chloride, uniformly distributed or carried on a fluidizable support, which is a particulate material of the proper ratio of particle sizes, surface area, porosity, density, resistant to attrition and other characteristics to provide proper fluidization and isothermal conditions in the reactor bed, to permit adequate contact between the copper catalyst and the gaseous reactants as they pass through the bed, and to minimize loss of catalyst through passage of fine particles from the reactor with the effluent gases. The fluidizable support, as taught by the Harpring et al patent, is composed of alumina, most desirably activated alumina or microgel alumina since such supports exhibit superior resistance to attrition and ability to fluidize and can be readily prepared to have the desired surface area and ratio of particle sizes in accordance with the "Bayer process" or other bauxite calcination techniques well known to the art. Preparation of the copper catalyst on the fluidizable support is well known to the art, and is described in the referenced Harpring et al patent. Typically cupric chloride is dissolved in water, and the solution is slowly sprayed on the support with continuous mixing (or alternatively adding the support to the solution with mixing) followed by drying the wet subject until it is free flowing, calcining for a few hours at a temperature of about 110.degree. C., and screening to eliminate large particles. The supported catalyst is then ready for addition to the oxyhydrochlorination reactor to function as the fluidized catalyst bed. The supported catalyst is prepared to contain from about 2 to 10 percent by weight copper. It is customarily supplied to the operator of the ethylene oxyhydrochlorination process for addition to the reactor at "start-up" of the process or as "make-up" when the catalyst bed needs replenishing.
Copper catalysts prepared as above have serviced well as oxyhydrochlorination catalysts. However, all such catalysts exhibit, to a more or less degree, a tendency to agglomerate, a characteristic which is called "stickiness" in the trade. The degree of stickiness of the catalyst is dependent on many factors, including the pressure and temperature of the reaction, the absorptive nature or porosity of the catalyst, the amount and distribution of the copper on the particle surfaces, the ratio of the weight of copper to the surface area of the support, the number of active sites available on the catalyst and the manner and degree of their utilization, the presence of contaminants such as sulfur, as well as upon the quantity and ratios of the gaseous reactants in the fluid bed. A certain degree of stickiness is tolerable, but if the catalyst is so sticky that particles continually agglomerate and are not broken up by movement in the fluidized bed, "hot spots" are developed in the bed at the point of the agglomeration, especially at the bottom of the bed. These hot spots may eventually lead to loss of fluidity or "inversion" and total collapse of the fluidized bed. Even if inversion does not occur, agglomeration of the catalyst can cause plugging of the lower portion or "dip leg" of the cyclone above the reactor (indicated by reference numeral 14 of FIG. 1a of the drawing of Harpring et al U.S. Pat. No. 3,488,398 which cyclones separate the catalyst fines from the effluent gases and retain the catalyst in the bed) with the result that large quantities of the catalyst can be lost and operations disrupted. To a large extent, the stickiness of the copper catalyst can be controlled by efficient operation of the process. However, an effective and practical way to inhibit and/or reduce stickiness of the catalyst during operation would be desirable.